Also knowing about how CGI works - they are in no way locked into THIS design for the final film - regardless of who it is suppose to be. The very nature of CGI allows them to make radical changes long after these scenes have been shot, limited only by time and money. They also do not "need" a motion capture performance - or at least one shot at this time - if it is Ultron perhaps his mo-cap will be shot later.

Agreed that about all we can assume for sure is that this character is standing in for something shiny and metallic.

Its possible that they can change the design in post, but at this point its unlikely. Why build something that is going to be different from the final product? They're not locked into using it, but right now at least they intend on keeping it.